Dude, this is something to be asking your doctor. Seriously. I've gassed out loads of times but the worst thing to happen to me is I went green and puked. Losing consciousness from exercise is not something to be taken lightly and needs to be investigated by a medical professional.

From time to time I get whiteies (cold sweat, enfeblement) while out walking. I can push through the whitie wall, but don't feel normal for maybe half an hour, and feel ravenouse for sweet stuff. Low blood sugar maybe?
The doc did my bloods but found nothing abnormal

I'm asthmatic myself and this does not sound like asthma. I go along with the previous advice but seriously don't sneak off to faint, tell the instructor that you feel faint just in case one day he/she finds you unconcious on the floor and doesn't know what the hell happened.

From time to time I get whiteies (cold sweat, enfeblement) while out walking. I can push through the whitie wall, but don't feel normal for maybe half an hour, and feel ravenouse for sweet stuff. Low blood sugar maybe?
The doc did my bloods but found nothing abnormal

Disconcerting

Sounds like anxiety symptoms. I used to get this when I was really stressed out at work and not sleeping properly, it would come on all of a sudden during non stressful activities like driving the car or watching tv at night. It was quite worrying at the time but I was able to get it sorted after talking to my GP. Just something worth thinking about.